In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. A transceiver module generates amplitude and/or phase and/or polarization modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver. The transceiver module includes a transmitter side and a receiver side. On the transmitter side, a laser light source generates laser light and an optical coupling system receives the laser light and optically couples, or images, the light onto an end of an optical fiber. The laser light source typically comprises one or more laser diodes that generate light of a particular wavelength or wavelength range. The optical coupling system typically includes one or more reflective elements, one or more refractive elements and/or one or more diffractive elements.
In high-speed data communications networks (e.g., 10 Gigabits per second (Gb/s) and higher), multimode optical fibers are often used. In such networks, certain link performance characteristics, such as the link transmission distance, for example, are dependent on properties of the laser light source and on the design of the optical coupling system. Among the most dominant ones are the modal bandwidth of the fiber and the relative intensity noise (RIN) of the laser light source, which can be degraded by the optical back-reflection to the laser light source. Both of these parameters can be affected by the launch conditions of the laser light into the end of the multimode optical fiber.
The effective modal bandwidth of multimode fiber is dependent in part upon the launch conditions of the laser light into the end of the fiber. The launch conditions are, in turn, dependent upon the properties of the laser diode itself and upon the optical coupling system design and configuration. However, due to limitations on the manufacturability of optical elements that are typically used in imaging-type optical coupling systems, control of the launch conditions is limited primarily to designing and configuring the optical coupling system to control the manner in which it images the light from the laser source onto the end of the fiber. Other types of non-imaging optical coupling system designs exist, such as spiral launch designs, for example, that overcome certain disadvantages of the imaging-type optical coupling systems. Such non-imaging systems, however, also have shortcomings.
Vertical Cavity Surface Emitting Laser Diodes (VCSEL)-based multimode optical communication systems have been considered low cost solutions due to the relative ease of optical coupling system designs. However, the increasing data rate of optical communication requires better noise control in the transmission link. Noise types include back reflection, RIN, mode partitioning noise (MPN), mode selective noise (MSN), etc. Back reflection from the optical surfaces of the optical coupling system destabilizes the VCSEL and increases the RIN. By adding perturbation to the perfect optical surfaces, back reflection can be mitigated. For example, U.S. Pat. No. 9,841,571 (hereinafter referred to as “the '571 patent”), which issued on Dec. 12, 2017 and which is owned by the assignee of the present application, discloses such perturbations and their effects. DOEs created using computer generated holograms (CGHs) can also be used to suppress optical feedback, in a more controlled and systematic manner. For example, U.S. Pat. No. 8,019,233 (hereinafter referred to as “the '233 patent”), which issued on Sep. 13, 2011 and which is hereby incorporated by reference herein in its entirety, discloses selecting a CGH that will achieve one or more a target launch conditions and creating a DOE that implements the CGH. Such DOEs can be used to suppress optical feedback.
There is a constant exchange of power between VCSEL modes and such exchanges can become the source of modal noise. When mode selectivity occurs in the optical coupling system, the coupled power will fluctuate, generating MSN. To overcome or reduce this noise source, the focus patterns generated by each VCSEL mode should overlap as much as possible spatially and the fiber modes excited should overlap as well so that mode selectivity is minimized at the connector joint. This effect can best be achieved by using a DOE designed to achieve this effect, as disclosed in the '233 patent.
MPN also surfaces through the combination of mode power exchange and the fiber modal dispersion. With a 50-micrometer (μm) graded index multimode fiber, VCSEL modes are typically coupled into the different fiber mode groups, which travel at different speeds in the fiber causing modal dispersion. When the power exchanges between VCSEL modes, MPN appears on the transmitted signals at any given time due to the time separation of fiber modes. To reduce this noise effect, the VCSEL modes should be mapped into the same fiber mode groups as evenly as possible so that at any given time, the transmitted signal contains all of the VCSEL mode content. Again this can only be best achieved by using a CGH to create a DOE.
DOEs that are designed to implement a selected CGH can be made in many different ways. A binary optics-based DOE made through a lithographic process is the most widely used DOE. The DOE is typically made on a glass substrate and the packaging of the DOE is a relatively complex design compared to plastic optics, which are often made using an injection molding process and can incorporate necessary mechanical features. DOEs are sometimes made of plastic, but plastic DOEs are not capable of achieving the same, or nearly the same, functionality as that of glass DOEs due to the inability to form the complex features of the diffractive pattern in plastic using current plastic fabrication techniques (i.e., injection molding and replication from a master mold). Such DOEs are typically either a Fresnel lens type of device having radially symmetric features or of the type having simple analytically describable surface molded in low temperature materials. It is difficult to replicate the features from a glass DOE used in transceiver design in injection molding, as the plastic material used for fiber optics applications is typically high temperature material, such as Ultem polyetherimide (PEI),
A need exists for a method of forming a plastic DOE that is capable of achieving the same, or nearly the same, functionality as that of a glass DOE in high temperature materials.